In general, such a spark plug includes a rodlike center electrode; a tubular insulator covering the outer circumference of the center electrode; a tubular metallic shell fitted onto the outer circumference of the insulator; and a ground electrode whose one end is welded to the front end surface of the metallic shell and whose other end is disposed to face the distal end of the center electrode to thereby form a spark discharge gap between the ground electrode and the center electrode.
In such a spark plug, in recent years, the ground electrode (also called “outer electrode” among persons in the spark plug industry) becomes more likely to suffer problems, such as breakage, because of an increase in the output of an internal combustion engine. Conceivable causes of such a problem include resonance and large acceleration (G) caused by the engine or combustion vibration. Also, occurrence of such a problem deeply relates to the structure in which the ground electrode is bent such that its distal end faces the center electrode and a bending moment is therefore apt to act on the proximal end of the ground electrode through which the ground electrode is attached to the metallic shell, and the structure in which the ground electrode is attached to a position where the ground electrode directly receives a shock wave or the like produced as a result of combustion.
In the above-described spark plug, increasing the cross-sectional area of the ground electrode is effective for improving the breakage resistance of the ground electrode. However, in the case where the cross-sectional area of the ground electrode is increased by increasing the width of the ground electrode, the flame-cooling effect of the ground electrode becomes stronger, and the ignition performance of the spark plug deteriorates.
Therefore, the cross-sectional area of the ground electrode is increased by increasing the thickness of the ground electrode. However, if the thickness of the ground electrode is rendered greater than the wall thickness of the metallic shell measured on the front end surface thereof, a portion of the ground electrode in the thickness direction thereof projects from the front end surface of the metallic shell in the radial direction, which may decrease the welding strength of the ground electrode.
In view of the above, in Patent Document 1, there is proposed a technique of rendering the thickness of the ground electrode equal to the wall thickness of the metallic shell measured on the front end surface thereof, and curving the ground electrode such that the cross-sectional shape of the ground electrode coincides with the curved shape of the front end surface of the metallic, to thereby increase the cross-sectional area of the ground electrode without lowering the welding strength, which would otherwise occur due to the projection in the radial direction.